Ni-based alloys have been widely applied as materials for members used in aircraft, gas turbines and the like, as disclosed by Patent Documents 1 to 3, for example.
In a gas turbine, a combustor is arranged at the back of a compressor and at a location closer to the periphery hereof, which functions to inject fuel into air discharged from the compressor, generate high-temperature and high-pressure gas for driving the turbine by combusting the fuel, and guide the fuel gas into nozzles (vanes) arranged at the inlet of the turbine, i.e., gas turbine combustors are used in high temperature environments.
In particular, the liner (inner cylinder) and the transition piece (tail pipe) among members and parts of a combustor are exposed to high-temperature combustion gases. In addition, the liner and the transition piece are subjected to frequent heating/cooling cycles in which heating and cooling are very frequently repeated for starting, stopping, and controlling the outputs of the gas turbine.
To consider the use conditions mentioned above, it is desired that Ni-based alloys used for the combustor and the like of gas turbines have high-temperature strengths such as high-temperature tensile strength, creep rupture strength, low-cycle fatigue strength, and thermal fatigue strength, excellent high-temperature corrosion resistance such as high-temperature oxidation or sulfurization resistance, and high cold workability, machinability, weldability, brazing characteristics. Similarly in aircraft and the like, the above-described use environment applies, and thus, the characteristics described above are required.
In such Ni-based alloys, in order to secure the above-described characteristics, it is required to strictly control the composition components and the metal structure, and also stocks to be charged are highly restricted. These restrictions are applied because the above-described characteristic may reduce due to the presence of inclusions in a Ni-based alloy such as nitrides and oxides. In particular, nitrides have been known to more remarkably influence various characteristics as their size becomes larger, and nitrides in which Ti is included as the main component of the metal components have been recognized to be harmful. Specifically, nitrides may become initiation points of cracks occurring due to creeps and creep fatigues caused during the use of gas turbines and thus reduce the life of Ni-based alloy members, and also considerably reduce the life of tools due to abnormal wear and chippings of cutting tools that occur during cutting.
In this regard, for example, Patent Document 2 discloses that the amount of nitrogen present in a Ni-based alloy should be confined to 0.01% by mass or less.
Patent Document 3 discloses that the maximum particle size of carbides and nitrides should be confined to 10 μm or less. It is pointed out that if the particle size of carbides and nitrides exceeds 10 μm, cracks may occur from the interface between the carbides or nitrides and the matrix phase during working at room temperature.
Furthermore, it is known in the field of iron and steel industry that a method for evaluating inclusions in Fe—Ni alloys such as Fe-36% Ni alloy and Fe-42% Ni alloy is carried out by estimating the maximum particle size of nonmetal inclusions, especially oxides, as disclosed in Patent Documents 4 and 5.